1. Field of the Invention
The present invention relates to a method, device, and system for evaluating the characteristics of an optical fiber transmission line, such as a loss at an optical fiber connection point.
2. Description of the Related Art
Methods of manufacturing and using a low-loss (e.g., 0.2 dB/km) optical fiber (e.g., silica fiber) have been established, and an optical fiber transmission system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber transmission line, one or more optical amplifiers for amplifying an optical signal are arranged along the optical fiber transmission line.
An optical amplifier known in the art includes an optical amplifying medium for propagating an optical signal, and a pumping source for pumping the optical amplifying medium so that the optical amplifying medium provides a gain in the optical signal. For example, an erbium doped fiber amplifier (EDFA) has an erbium doped fiber (EDF) as the optical amplifying medium, and the EDF is pumped by a pump light having a wavelength included in a 0.98-xcexcm band or a 1.48-xcexcm band.
In recent years, Raman amplification is sometimes utilized to obtain an additional gain in a system having an optical amplifier in the middle of an optical fiber transmission line. In general, when light having large power is supplied to an optical fiber, a relatively wide gain band is generated by the effect of Raman amplification at wavelengths longer than the wavelength of the light. For example, when light having a wavelength included in a 1.45 to 1.48 xcexcm band and having a power larger than +0 dBm is supplied to a silica fiber, a gain band providing a gain of about 0.1 to 8 dB is generated at wavelengths longer than t he wavelength (1.55-xcexcm band) of in put light by 0.009 to 0.10 xcexcm.
In improving the efficiency of generation of Raman amplification, it is effective to increase the power of light input into an optical fiber or to use an optical fiber having a small mode field diameter. Conversely, the gain by Raman amplification changes with a change in either the power of input light or the mode field diameter of an optical fiber.
It is difficult to accurately grasp the mode field diameter of an optical fiber used as an optical fiber transmission line from the viewpoint of a method of manufacturing . Further, the power and wavelength of light input to an optical fiber transmission line to produce Raman amplification differ according to systems. Accordingly, in the case of evaluating the characteristics of an optical fiber transmission line, such as a loss at a splice connection point on the optical fiber transmission line, there is a possibility of large errors due to variations in gain by Raman amplification.
In the case that the loss by splice connection is high, the connection of optical fibers may be imperfect, causing a possibility of breaking of the transmission line due to aged deterioration or shock against a cable.
Such imperfect connection of optical fibers causes a fatal defect in a system required to have the long-term reliability of a cable, such as in a submarine communication system.
It is therefore an object of the present invention to provide a method, device, and system for accurately evaluating the characteristics of an optical fiber transmission line.
In accordance with the present invention, there is provided a first method for evaluating the characteristics of an optical fiber transmission line. In this method, an optical amplifier comprising a pumping source for supplying pump light to the optical fiber transmission line is provided so that at least a part of the optical fiber transmission line produces Raman amplification to an optical signal. An optical filter unit for selectively switching between a first condition where the optical signal and the pump light are passed and a second condition where the optical signal is passed and the pump light is not passed is connected between the optical fiber transmission line and the optical amplifier. The power of the optical signal in the first condition is compared with that in the second condition, and the characteristics of the optical fiber transmission line are evaluated according to the result of this comparison.
According to this method, Raman amplification does not occur in the second condition, so that a measurement error due to variations in gain by Raman amplification can be eliminated to thereby attain the object of the present invention.
In accordance with the present invention, there is provided a device suitable for carrying out the first method. This device comprises an optical amplifier comprising a pumping source for supplying pump light to an optical fiber transmission line so that at least a part of the optical fiber transmission line produces Raman amplification to an optical signal; an optical filter unit connected between the optical fiber transmission line and the optical amplifier for selectively switching between a first condition where the optical signal and the pump light are passed and a second condition where the optical signal is passed and the pump light is not passed; and a control circuit for controlling the optical filter unit so as to switch between the first condition and the second condition.
In accordance with the present invention, there is provided a system suitable for carrying out the first method. This system comprises an optical fiber transmission line for propagating an optical signal; an optical amplifier comprising a pumping source for supplying pump light to the optical fiber transmission line so that at least a part of the optical fiber transmission line produces Raman amplification to the optical signal; an optical filter unit connected between the optical fiber transmission line and the optical amplifier for selectively switching between a first condition where the optical signal and the pump light are passed and a second condition where the optical signal is passed and the pump light is not passed; and a control circuit for controlling the optical filter unit so as to switch between the first condition and the second condition.
In accordance with the present invention, there is provided a second method for evaluating the characteristics of an optical fiber transmission line. In this method, first and second optical amplifiers are connected to a first end and a second end of the optical fiber transmission line, each of the first and second optical amplifiers comprising a doped fiber (e.g., EDF) doped with a rare earth element, a first pumping source connected to a first end of the doped fiber for outputting first pump light, and a second pumping source connected to a second end of the doped fiber for outputting second pump light. Switching is performed between a first condition where the first and second pumping sources of the first optical amplifier are turned off and on, respectively, and the first and second pumping sources of the second optical amplifier are turned on and off, respectively, and a second condition where the first and second pumping sources of the first optical amplifier are turned on, and the first and second pumping sources of the second optical amplifier are turned on. The power of the optical signal in the first condition is measured, and the characteristics of the optical fiber transmission line are evaluated according to the result of this measurement.
According to this method, a measurement error due to variations in gain by Raman amplification can be eliminated in the first condition, thereby attaining the object of the present invention.
In accordance with the present invention, there is provided a device suitable for carrying out the second method. This device comprises first and second optical amplifiers each comprising a doped fiber doped with a rare earth element, a first pumping source connected to a first end of the doped fiber for outputting first pump light, and a second pumping source connected to a second end of the doped fiber for outputting second pump light; and a control circuit for switching between a first condition where the first and second pumping sources of the first optical amplifier are turned off and on, respectively, and the first and second pumping sources of the second optical amplifier are turned on and off, respectively, and a second condition where the first and second pumping sources of the first optical amplifier are turned on, and the first and second pumping sources of the second optical amplifier are turned on, according to a supervisory signal.
In accordance with the present invention, there is provided a system suitable for carrying out the second method. This system comprises first and second optical amplifiers each comprising a doped fiber doped with a rare earth element, a first pumping source connected to a first end of the doped fiber for outputting first pump light, and a second pumping source connected to a second end of the doped fiber for outputting second pump light; an optical fiber transmission line having a first end and a second end respectively connected to the first and second optical amplifiers; and a control circuit for switching between a first condition where the first and second pumping sources of the first optical amplifier are turned off and on, respectively, and the first and second pumping sources of the second optical amplifier are turned on and off, respectively, and a second condition where the first and second pumping sources of the first optical amplifier are turned on, and the first and second pumping sources of the second optical amplifier are turned on, according to a supervisory signal.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.